(a) Field of the Invention
The present invention relates to a single-lens reflex optical system for endoscopes.
(b) Description of the Prior Art
To observe the inside of tubular members such as a rectum, esophagus, etc., a forward-viewing type endoscope is used. This is due to the following reason. If a side-viewing type endoscope is used for observation of a tubular member, the object to be observed or photographed (the inner wall of the esophagus or the like) comes into close contact with the cover glass arranged at the front of the objective of the endoscope and, consequently, it becomes impossible to observe or photograph the object with an adequate magnification and field.
FIG. 1 shows an example of known single-lens reflex optical systems to be used in a forward-viewing type endoscope, which is used for observation of a tubular member and which is arranged so that a film cassette can be loaded. The optical system shown in FIG. 1 comprises a photographing optical system and an observing optical system, the photographing optical system comprising lens components 2, 6 and 7 arranged on the optical axis 1 which extends in parallel with the longitudinal direction of the endoscope, the observing optical system comprising lens components 2, 3 and 4, the lens components 3 and 4 being arranged on the optical axis 1' which is split by a light-splitting prism 9 and directed to an optical fiber bundle 5. In case of the above-mentioned optical system, a film surface 8 is arranged perpendicular to the optical axis 1. Therefore, the film cassette should be loaded in the direction perpendicular to the longitudinal direction of the endoscope. As, however, the film cassette is very long compared with the diameters of lens components, the diameter of the distal end of the endoscope becomes large when the film cassette is arranged in the direction perpendicular to the longitudinal direction of the endoscope.
As a method to eliminate the above-mentioned disadvantage, it may be considered to arrange the film cassette in the direction parallel with the longitudinal direction of the endoscope. FIG. 2 shows an optical system in which the above-mentioned idea is adopted. The optical system shown in FIG. 2 comprises an observing optical system and a photographing optical system, the observing optical system comprising a diverging front lens group 11 and a first converging rear lens group 12 which are arranged on the optical axis 10, the photographing optical system comprising the diverging front lens group 11 and a second converging rear lens group 14, the second converging rear lens group 14 being arranged on the optical axis 10' which is split by a light-splitting prism 13.
In case of the optical system shown in FIG. 2, the space in the diametral direction in the distal end of the endoscope which is occupied by the film cassette 15 becomes very small compared with the case of the optical system shown in FIG. 1. However, in case of a retrofocus-type optical system for endoscopes as shown in FIG. 2, the airspace between the diverging front lens group and each of the converging rear lens groups becomes large when the film cassette is arranged in the longitudinal direction of the endoscope. In case of a retrofocus-type optical system, paraxial rays are diverged by the diverging front lens group and, consequently, the height of the paraxial marginal ray which enters the converging rear lens group becomes larger when the airspace between the front and rear lens groups becomes larger. Therefore, when the airspace between the diverging front lens group and converging rear lens group becomes larger, diameters of lenses in the converging rear lens group should be made larger. Besides, in order to let the offaxial ray enter the coverging rear lens group at a pre-determined angle, the height of the offaxial ray which enters the diverging front lens group should be made larger when the airspace between the diverging front lens group and converging rear lens group becomes larger. Consequently, it becomes necessary to make the diameter of the lens constituting the diverging front lens group larger. As explained in the above, when the airspace between the diverging front lens group and converging rear lens group becomes large due to the fact that the film cassette 15 is arranged in the longitudinal direction of the endoscope, diameters of lenses in both of the diverging front lens group and converging rear lens group become large. In other words, in spite of the fact that the space in the distal end of the endoscope occupied by the film cassette 15 becomes smaller, diameter of lenses become larger and, consequently, the diameter of the distal end of the endoscope becomes larger also in case of the optical system shown in FIG. 2.